docs: def add_constants(namespace, registry):
      """Adds the quantities from :mod:`unyt.physical_constants` to a namespace
  
      Parameters
      ----------
  
      namespace : dict
         The dict to insert quantities into. The keys will be string names
         and values will be the corresponding quantities.
      registry : :class:`unyt.unit_registry.UnitRegistry`
         The registry to create units from. Note that if you would like to
         use a custom unit system, ensure your registry was created using
         that unit system.
  
      Example
      -------
      >>> from unyt.unit_registry import UnitRegistry
      >>> class MyClass():
      ...     def __init__(self):
      ...         self.reg = UnitRegistry(unit_system='cgs')
      ...         add_constants(vars(self), self.reg)
      >>> foo = MyClass()
      >>> foo.gravitational_constant
      unyt_quantity(6.67384e-08, 'cm**3/(g*s**2)')
      >>> foo.speed_of_light
      unyt_quantity(2.99792458e+10, 'cm/s')
      """
      from unyt.array import unyt_quantity
  
      for constant_name in physical_constants:
          value, unit_name, alternate_names = physical_constants[constant_name]
          for name in alternate_names + [constant_name]:
              quan = unyt_quantity(value, unit_name, registry=registry)
              try:
                  namespace[name] = quan.in_base(unit_system=registry.unit_system)
              except UnitsNotReducible:
                  namespace[name] = quan
              namespace[name + "_mks"] = unyt_quantity(
                  value, unit_name, registry=registry
              )
              try:
                  namespace[name + "_cgs"] = quan.in_cgs()
              except UnitsNotReducible:
                  pass
              if name == "h":
                  # backward compatibility for unyt 1.0, which defined hmks
                  namespace["hmks"] = namespace["h_mks"].copy()
                  namespace["hcgs"] = namespace["h_cgs"].copy()